The present invention, in some embodiments thereof, relates to calibration and, more specifically, but not exclusively, to automatic firearm sight calibration, also known as firearm zeroing.
Over the years, various techniques and devices have been developed to help a person accurately aim a firearm, such as a rifle or target pistol. One common approach is to mount on the firearm's barrel a sight or scope, through which the person views the intended target in association with a reticle, often with a degree of magnification. Although existing firearm sights have been generally adequate for their intended purposes, they have not been satisfactory in all respects.
For example, when a sight is first mounted on the barrel and/or body of a firearm, it needs to be aligned or “zeroed” with the firearm barrel, typically through a trial and error process. For example, a person may shoot one or more bullets at a target which is a known distance away, identify the extent to which the bullets strike the target at locations offset from the location at which the person was aiming, and then adjust the alignment of the sight in relation to the firearm in a manner intended to eliminate the offset. This sequence of steps is repeated in an iterative manner, until bullets are striking the target at substantially the same location where the person is aiming.
This process results in alignment of the sight and firearm and needs of a shooter for one specific set of conditions. However, during subsequent use of the firearm and sight, for example when hunting, a variety of conditions can vary from the conditions that existed during the alignment or zeroing process, and can thus affect the trajectory of a bullet. These include factors such as temperature, pressure, humidity, wind speed and wind direction, all of which affect the density of air and thus the drag exerted on the bullet, and drag in turn influences the trajectory. Further, the tilt of the firearm barrel can influence the direction in which gravity acts on the bullet in relation to the initial trajectory of the bullet, and this can in turn influence how gravity
Firearms such as rifles and handguns have adjustable sights. In M16A2 Rifle for example elevation adjustments are made using the front sight, and elevation changes and windage adjustments are made using the rear sight.
In M16A2 Rifle the rear sight has an elevation knob with range indicators from 300 to 800 meters and two apertures for range. One aperture is marked 0-2 for short range from 0-200 meters and an unmarked aperture for normal range from 300 to 800 meters. The 0-2 (large) aperture is used for short range when the rear sight is all the way down. The 8/3 (300-meter) mark on the elevation knob is aligned with the index mark on the left side of the receiver. The rear sight also consists of a windage knob on the rear side of the sight. Each desecrate turning action of the windage knob, known as a click, moves a strike on a round from ⅛ inch (0.3 centimeters) at 25 meters to 4 inches (10 centimeters) at 800 meters. In M16A2 Rifle a windage scale is on the rear of the sight and the windage knob pointer is on the windage knob. To adjust windage or move the strike of the round, the windage knob is turned counterclockwise to move the strike to the left and clockwise to move the strike to the right. To adjust elevation the elevation knob is turned until the desired range is indexed at the index mark on the left side on the sight.
In order to calibrate the rear sight a mechanical zero is usually established on the rifle and then a target such as 25 meters zeroing target (e.g. see FIG. 1A) is used on shooting sessions with the firearm to be calibrated. After each shooting session a shot group is manually identified in the target, for example a shot group of three bullet holes. When the shot group is not within a defined region such as a circle on a silhouette of the target, squares on the target are used to determine the required adjustment. The squares are usually numbered around the edges of the target to equal the number of clicks required to relocate the shot group to the circle. The number of squares may be determined in relation to a center of the bullet holes which comprise the shot group is identified. The center may be calculated by marking a first line between bullet holes of one pair of bullet holes and then marking a second line between a third bullet hole and the center of the first line. This allows manually marking a center of the second line as the center of a shot group that comprises 3 bullet holes.
Additional shooting sessions are continuously held to form a sequence of shot groups and making corrections between the shooting sessions until a shot group is identified in the circle on the silhouette. Calibration is achieved when a shot group is within a target region such as the circle. Calibration is achieved when the shot group within the target region is tight, for example holes are not located at a distance of more than few centimeters from, one another.